Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobberedunnecessarily

[Linus Torvalds]

On Tue, 24 Jul 2007, Benjamin Herrenschmidt wrote:
> 
> In fact, it's more than that... the bitops that return a value are often
> used to have hand-made spinlock semantics. I'm sure we would get funky
> bugs if loads or stores leaked out of the locked region. I think a full
> "memory" clobber should be kept around for those cases.

Not helpful.

The CPU ordering constraints for "test_and_set_bit()" and friends are weak 
enough that even if you have a full memory clobber, it still wouldn't work 
as a lock.

That's exactly why we have smp_mb__after_set_bit() and friends. On some 
architectures (arm, mips, parsic, powerpc), *that* is where the CPU memory 
barrier is, because the "test_and_set_bit()" itself is just a 
cache-coherent operation, not an actual barrier.

		Linus
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